1. Field of Invention
This invention relates to porous dielectric films having a dielectric constant (k) of less than about 2.5. Over the past years there has been a constant drive to produce dielectric materials having low dielectric constants for use, particularly, in semiconductor devices to accommodate the ever decreasing dimensions of the device architecture. It is presently believed that to achieve k values of less than about 2.5 for a practical insulator, there is inevitably a degree of porosity in these materials. This porosity can present major problems for integration, particularly when vias or interconnects are formed through the dielectric layer, because, when etched, the side walls of the etched formations are at least rough and possibly permeable, if the pores interconnect at all and intersect the surface.
It is into these etched trenches and vias that copper is deposited, in typical present day architectures, and because copper would readily diffuse into the dielectric material it must be contained by a diffusion barrier. An ideal would be an insulator that had barrier characteristics, but present day solutions rely upon separate deposited layers.
2. Description of the Related Art
Traditionally these barrier layers were deposited using physical vapour deposition techniques, but these techniques struggle to provide sufficient conformity of barrier coverage and so chemical vapour deposition (CVD) techniques are used e.g. Metal Organic CVD, Metal Halide CVD and Atomic Layer CVD. Whilst CVD techniques can give near 100% conformity, the precursors and reactants can penetrate the porous dielectric. This effect is shown in FIG. 1 (Source IMEC at the ARMM 2001 Conference) and FIG. 2 (Source Passemard et al; “integration issues of low k and ULK materials and damascene structure” at CREMSI 2001 Conference). In both cases, it will be seen that the side wall appears “fuzzy” and this indicates that CVD precursors have been absorbed into the porous dielectric layer giving rise to an indistinct barrier between the dielectric and the barrier layer.
EP-A-1195801 describes processes which are in fact believed to increase the porosity of the side walls and proposes sealing pores created from the side walls by providing a protective or sealing layer. It suggests that such a sealing layer can be formed by a plasma comprising oxygen and nitrogen, but gives no substantive description of the process. The addition of extra material into high aspect ratio vias is undesirable, both because it increases the aspect ratio of the via and may increase the resistance of the copper in the via. It is not clear whether or not the sealed surface maintains the local low k values as suggested in the application.